The present invention relates to an apparatus and a method of generating a carburizing atmosphere, and more particularly to an apparatus and a method which can generate a carburizing atmosphere having a composition that allows a carburizing treatment to be effectively performed on steel components and the like.
The air mixing method is widely used as a method of generating a carburizing atmosphere containing carbon monoxide and hydrogen. In the air mixing method, paraffin hydrocarbon such as LNG or LPG is mixed with the air, and the mixed gas (raw material mixed gas) is introduced into a reforming furnace having a nickel catalyst bed which is held at a high temperature. A catalytic reaction (reforming reaction) of oxygen in the air with hydrocarbon is performed, so as to obtain a reformed gas containing carbon monoxide and hydrogen.
In the air used as an oxygen source, however, about 79% nitrogen (vol. %, the same shall apply hereinafter) exists. Therefore, the concentrations of carbon monoxide and hydrogen in the obtained reformed gas cannot be increased to be equal to or higher than a certain degree. In the case of using methane, for example, the maximum concentration of carbon monoxide is 20%, and, in the case of using butane, the maximum concentration is 23.5%.
On the other hand, in the carburizing treatment, particularly in the high-temperature rapid carburizing treatment, if the concentration of carbon monoxide is low, a stable carburizing atmosphere cannot be attained in the carburizing furnace, because of gas equilibrium at a high temperature. For this reason, the concentration of carbon monoxide in the atmosphere is required to be high. Such a high concentration of carbon monoxide can attain the following advantages. In the case where a component having a hole is subjected to the carburizing treatment, for example, the whole interior of the hole can be sufficiently uniformly carburized. In the case where minute components which are piled up and carried on a belt conveyor are subjected to the carburizing treatment, the piling-up height of the components on the belt can be increased.
As a method of increasing the concentration of carbon monoxide in a reformed gas, known is a method in which carbon dioxide or oxygen is used instead of the air as a source gas that is to be mixed with hydrocarbon. Theoretically, when a reforming reaction is performed with the mole ratio of methane to oxygen of 2:1, 2 moles of carbon monoxide and 4 moles of hydrogen are generated. Thus, a reformed gas in which the concentration of carbon monoxide is about 33.3% and that of hydrogen is about 66.7% is obtained. Similarly, when the reaction is performed with the mole ratio of methane to carbon dioxide of 1:1, 2 moles of carbon monoxide and 2 moles of hydrogen are generated, so that their concentrations are 50%, respectively. In the case of butane, as a result of the reaction with 2 moles of oxygen, 4 moles of carbon monoxide and 5 moles of hydrogen are generated, and, as a result of the reaction with 4 moles of carbon dioxide, 8 moles of carbon monoxide and 5 moles of hydrogen are generated.
When carbon monoxide of high concentration is to be generated by using carbon dioxide or oxygen, however, soot which is generated in the reforming furnace causes a serious problem. When carbon dioxide is used as a source gas, for example, the supply of heat from a heater which heats the nickel catalyst bed to a predetermined temperature is partly insufficient, because the reforming reaction is an endothermic reaction. If temperature reduction occurs in part of the nickel catalyst bed, the reaction does not sufficiently proceed in the part, and soot is generated. Also when the concentration of carbon dioxide in the generated gas is low, soot is generated because of the gas equilibrium of carbon monoxide and carbon dioxide. The generation of a large amount of soot in the reforming furnace causes the nickel catalyst bed to be clogged, so that the apparatus cannot be further operated.
On the other hand, in the case where oxygen is used, since the reforming reaction is an exothermic reaction, the problem related to the generation of soot caused by the temperature reduction does not occur. However, the temperature in the reforming furnace is extraordinarily raised because of the exothermic reaction of hydrocarbon with oxygen, thereby causing the possibility that the structural members are softened and deformed and the apparatus malfunctions, or that part of members is molten. Thus, there arise problems in safety.
As a result, although it is known that a reformed gas containing carbon monoxide of high concentration can be obtained by using carbon dioxide or oxygen as a source gas instead of the air, a conventional addition method in which, placing emphasis on the safety and stability, the air is used as the source gas is still adopted in an actual apparatus.